Starting the operation of a turbine engine is an essential, but also a difficult phase of operation, in particular in the case of relatively small engines. Most often, small turbine engines, e.g. auxiliary power units, are optimized to operate within a limited operational regime requiring airflow pressure and temperature that may be quite different from the actual conditions existing when the engine is started.
In broad terms, a turbine engine operates by ingestion of a mass of external air through an intake section disposed upstream in the fore portion of the engine. The ingested air is compressed by a rotating compressor, which is positioned in a compressor section accommodated downstream of the air intake. The compression process increases the pressure and the temperature of the ingested air.
The compressed air then flows downstream into a combustion section including one or more combustion chambers, in which fuel is injected and atomized or vaporized, and mixed with a portion of the ingested air to form a fuel-air mixture. Another portion of the air is gradually introduced into the combustion chamber(s) to dilute and cool the gases produced by the combustion process. Next, the fuel-air mixture is ignited, whereby the temperature increases. The diluted combustion gases then flow downstream into a turbine section having a turbine, which is thereby rotated. Typically, the turbine is rotationally coupled to the compressor by a common shaft supported by appropriate bearings. Finally, the exhaust gases flow downstream through and out of the turbine into an exhaust section. The resulting energy, in the form of the shaft rotation or the exhaust gas velocity, is used for driving a power-consuming unit, or for producing thrust.
A turbine engine is supposed to include the various portions mentioned hereinabove, and also includes further ancillary equipment, not mentioned hereinabove, necessary for proper operation of the engine.
Before functioning properly to provide the expected power or thrust, the turbine engine has first to be started and thereafter accelerated to a stable operational regime. Frequently, the starting process of a turbine engine is initiated when the pressure and temperature of the ambient air, and of the air in the combustion chamber(s), are much lower than the specific values for which the engine was designed. Under such adverse conditions, the combustion process may fail to produce the energy necessary for accelerating the turbine engine to a steady operational regime. Furthermore, harsh environmental conditions cause the starting process of a turbine engine to become difficult to achieve, if at all, unless additional provisions are provided.
Various proposals to improve the starting of a turbine engine under unfavorable circumstances have been cited. U.S. Pat. No. 5,231,822 by Shekleton, referred to hereinbelow as Shekleton, recites enhanced starting capability of a turbine engine by controllably bleeding compressed air from the airflow path at a variable rate by means of valves and ducts. Thereby, Shekleton adds components and weight in the form of peripheral equipment in order to improve the starting process.
U.S. Pat. No. 6,374,592 by Box et al., referred to herein below as Box et al., recites the addition of energy to a turbine engine in the form of supplemental heat provided by burning of solid fuel. Box et al. disclose a quantity of solid fuel or propellant that is attached to the inside of the combustion chamber of a turbine engine. When the engine begins a starting sequence, a starter sets the spinning of the compressor in motion, and thereby a flow of air is pumped through the engine. The solid fuel or propellant is then heated by an electric heater element until it begins to burn. The combustion of the solid fuel increases the temperature of the combustion chamber, so that when an atomized or vaporized liquid fuel is injected, the temperature within the combustion chamber is sufficient to allow the liquid fuel to burn in a self-sustaining manner. Box et al. thus dispose both an electrical heater and a quantity of solid fuel in the interior of the combustion chamber.
It is well known that the addition of oxygen to the combustion process drastically improves the starting process of turbine engines, as recited by Chen et al., in “Effect of Oxygen Addition on Ignition of Aero-Gas Turbine at Simulated Altitude Facility”, J. Energy, Vol. 6, No. 6, November-December 1982, pp. 425-429.
U.S. Pat. No. 6,941,760 by Jones, referred to hereinbelow as Jones, discloses a starting system for expendable turbine engines having a tank of gaseous oxygen pressurized to some 5000-7000 psig. Oxygen is communicated from the tank to a compressor through one passage to spin-up a rotor, and through another passage to provide light-off oxygen for the atomized fuel within the combustion chamber. Jones thus requires a bottle of oxygen at high pressure and associated accessories, which are rather voluminous and heavy and become a deadweight ballast after the completion of the starting process. The density of the molecular oxygen in the tank is 0.4-0.6 gram per 1 cm3, even without taking into account the volume of tank walls.
U.S. Pat. No. 4,161,102 by Jasas et al., referred to herein below as Jasas et al., recites a turbine engine having a starting system that comprises a separate auxiliary combustion chamber having a source of fluid combustible fuel and a source of fluid oxidizer, and appropriate fluid conduits to the main chamber. A portion of the hot exhaust gas exiting the auxiliary combustion chamber is preferably diverted and injected with supplemental oxygen into the main combustion chamber of the turbine engine, thereby initiating self-sustained combustion. Another portion is used to drive the main turbine and hence the rotor. Jasas et al. thus require a separate auxiliary combustion chamber in addition to the conventional combustion chamber(s), a source of fluid combustible fuel, a source of fluid oxidizer, and appropriate fluid conduits with all necessary hardware.
It would thus be advantageous to devise a method and means for enhancing the starting process of a turbine engine by raising the temperature inside the combustion chamber(s) and simultaneously providing additional oxygen to the combustion process. This, however, by provision of means disposed solely on the exterior of the turbine engine to permit fast and easy replacement of spent means without requiring the dismantling of the engine, and to allow retrofit of existing engines. The devised means should preferably be configured as a single solid-state unitary module void of moveable elements, generate oxygen in situ, supply heat, and not be pressurized for the sake of safe storage. Such means would allow, for example, running an Acceptance Testing Procedure (ATP) on a turbine engine, and thereafter replace the spent starting means for immediate return of the tested engine to full operational service.